The Second Amendment provides: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."
Given the reference to "a well regulated Militia" and "the security of a free State," the ACLU has long taken the position that the Second Amendment protects a collective right rather than an individual right. For seven decades, the Supreme Court's 1939 decision in United States v. Miller was widely understood to have endorsed that view. This position is currently under review and is being updated by the ACLU National Board in light of the U.S. Supreme Court decision in D.C. v. Heller in 2008.
Although ACLU policy cites the Supreme Court's decision in U.S. v. Miller as support for our position on the Second Amendment, our policy was never dependent on Miller. Rather, like all ACLU policies, it reflects the ACLU's own understanding of the Constitution and civil liberties.
Heller takes a different approach than the ACLU has advocated. At the same time, it leaves many unresolved questions, including what firearms are protected by the Second Amendment, what regulations (short of an outright ban) may be upheld, and how that determination will be made.
Those questions will, presumably, be answered over time.